Donald Trump described an allegation of sexual assault against his nominee for the US Supreme Court as "very unfair" as the alleged victim said there should be an FBI investigation.

At the same time, senior Republicans set a deadline of today for Christine Blasey Ford, a California psychology professor, to decide whether she will testify before senators.

She has accused Judge Brett Kavanaugh of pinning her on a bed and trying to take off her clothes during a high school party in 1982, when she was 15 and he was 17.

In an open letter, Dr Ford said she had received death threats since making her allegations public and had been forced to leave her home. She asked that the FBI carry out an investigation before she gives evidence to the Senate judiciary committee, which it has invited her to do on Monday.

Opinion polls showed support for Mr Kavanaugh sinking, with only 31pc backing his appointment to a seat for life on the US's highest court.

Speaking outside the White House, Mr Trump said: "Look, if she shows up and makes a credible showing that'll be very interesting. We'll have to make a decision. But I can only say this - he is such an outstanding man. Very hard for me to imagine that anything happened.

"Really they're hurting somebody's life. I think it's a very unfair thing what's going on. He has an unblemished record. This is a very tough thing for him and his family."

However, Mr Trump also said he wanted to hear from the alleged victim. He said: "I'd really want to see her. I really would want to see what she has to say. If she shows up, that would be wonderful. If she doesn't show up that would be unfortunate."

Republicans were hoping to secure Mr Kavanaugh's appointment this month, but a delay could push it beyond the mid-term elections in November.